Few situations are more uncomfortable than when you know your partner is making bad line calls. Here's a diplomatic means of handling the situation if it happens to you:

Next time you witness a what you feel is a bad call from your partner, wait until your opponents question the call. If they don't question the call it may be because the ball was indeed out. When they do question the call they will ask your partner if he is sure of his call. You remain quiet and see what happens. If your partner seems at all unsure, the opponents will suggest a replay of the point and you can agree.

If your partner's adamant about the ball being out, remain quiet and wait until your opponents ask you for your opinion. Then you can say: "My partner was in a better position to call it (if that was the case)- but I thought it MAY have been good. Then, since there is doubt, you can suggest you play the point over or give up the point. This tactful approach lets your partner save face (whether he deserves to or not!). If it's the type of match where your opponents would never embarrass your partner by questioning the call, you can just look at them and roll your eyes a bit. This way everyone knows what's going on and relationships are not damaged.

Tennis players are a dime a dozen. Friends are priceless.